(i) Field of Invention
The present invention broadly concerns improved apparatus and method for filtering molten polymeric material. More particularly, it relates to improved particulate material used for filtering polyesters such as poly(ethylene terephthalate) in melt-extrusion processes.
(ii) Prior Art
Melt-extrusion processes including filtration of molten polvmer by means of particulate filter media are extremely old in the art. For example, see U.S. Pat. Nos. 4,260,350; 3,896,028; 3,307,216; 3,044,628; 2,607,954; and 2,266,368.
At least some of the types of specific particulate filter media suitable for use in the present invention are also old in the art for other end uses. For example, see The Carborundum Company's "Sintered Bauxite High-Strength Proppant: Product Specifications" (March, 1980). The use of sintered bauxite as a high-strength proppant in deep wells is described in Carborundum's "High-Strength Proppant" (April, 1979). It is pointed out that Carborundum's product significantly outperforms such proppants as silica sand and glass beads, for a variety of reasons including better crush and fragmentation resistance.
Sintered bauxite had, apparently, never been used in the past for filtering molten polymer.
U.S. Pat. No. 2,607,954 (Celanese) discloses the use of ceramic particles in filtering molten cellulose acetate. Example V of the patent discloses the use of ground ceramic particles of 30/50 mesh, the particles being wet with diethyl phthalate as a plasticizer.
U.S. Pat. No. 3,896,028 discloses the use of coarse No. 10 aluminum oxide particles to "fill the remaining space" above metallic particles of 25/140 mesh size, for filtering molten poly(ethylene terephthalate) polymer.
There may have been public use of alumina particles for filtering molten polymers such as nylon and polyester. However, it is believed that any such public use has been limited to angular particles having a "roundness" far below 0.8, as determined by Krumbein and Sloss Chart of 1955. Applicant's assignee used commercially available Al.sub.2 O.sub.3 particles sold under the trademarks ALUNDUM and BAUXILITE for filtering nylon polymer in the 1960's. Such work was not known to Applicant when he conceived this invention. Assignee concluded that "sand has a more uniform configuration than Bauxilite and Alundum" and "(i)t is thought that the sharp edges on Alundum and Bauxilite tend to break off during the life of the pack, thereby building up fines which ultimately decrease pack life by plugging the channels through which the polymer flows". It is believed that these particles were "fused" rather than "sintered".
Iron oxides are well known as promoters of degradation leading to gel-formation in hot polymers. Various techniques have been used in the past in order to overcome this problem. In particular, see forementioned U.S. Pat. No. 3,896,028 at column 1, lines 29-49; column 3, lines 16-18; column 5, line 26 to column 6, line 9; column 6, lines 53-61; and column 8, line 56 to column 9, line 3.
Carborundum's U.S. Pat. No. 3,239,970 describes a process for making microcrystalline sintered bauxite tumbling pellets with cross sectional dimensions of 1/16 inch to 2 inches. Such pellets, when used as tumbling media, are as effective as more expensive pellets made from high purity alumina. (Bauxite, in addition to alumina, contains appreciable quantities of iron oxide, silica and titania.) Subsequently, in the last few years, Carborundum sold microcrystalline sintered bauxite in much smaller particle sizes than those used for tumbling pellets. Such product has been used, as previously indicated, as a high strength proppant in oil wells. It is understood that Carborundum's process for making the proppant is essentially similar to the process disclosed in U.S. Pat. No. 3,239,970, except that the comminuted bauxite is sintered directly and then screened.
U.S. Pat. No. 3,896,028 extends prior art teachings that "improvements in shear-filtration can result from the use of irregularly shaped metal particles". In the same context, "Particle Shape Factor", .phi., of some prior art particulate melt-filtration materials is discussed in forementioned U.S. Pat. No. 3,896,028, particularly at column 6, lines 19-44. Spherical particles (such as glass beads) have a Shape Factor of 1.0. Silica sand in mesh sizes commonly used has a .phi. of about 0.77, and consequently has low "porosity" (i.e., amount of free space) in the range of 30 to 40%. The patentee's irregular particles have initial porosity, E.sub.o, greater than 60%; and Shape Factor, .phi., of 0.26-0.58.
Accordingly, the prior art as a whole teaches away from filtering molten polymers with particles containing free iron oxide and/or having shapes approaching spherical.